1. Field of the Invention
The present invention relates to a computed tomography apparatus (hereinafter simply called "CT apparatus") which obtains the tomographic images of a subject body to be examined.
2. Description of the Related Art
CT apparatuses are very popular medical diagnosis apparatuses. The third generation CT apparatus, the currently popular type, has an X-ray tube for generating an X-ray fan beam, which is flat to match with each slice of a subject body and spreads in a fan shape in each slice, and an X-ray detector which has multiple small X-ray detection elements aligned in a line over the area where this fan beam spreads. The X-ray tube and the X-ray detector face each other with the subject body in between. As the X-ray tube and the X-ray detector as a pair are moved around a subject body, a fan beam is irradiated on the subject body. During the irradiation of the fan beam, the fan beam which has passed the subject body is detected to acquire projection data from various directions around a slice. The projection data of 360 degrees around a slice is subjected to some computation using a predetermined image reconstruction algorithm to obtain the tomographic image of the subject body.
To obtain a reconstructed image with high precision, it is necessary to equalize the detected values of all the detection elements. Therefore, in the conventional apparatus, there is provided a sub detector for detecting an X-ray beam which is not passed through the subject body in addition to a main detector for detecting the X-ray beam which is passed through the subject body. The outputs of the detection elements of the main detector are corrected in accordance with the output of the sub detector so that the outputs of the detection elements of the main detector are made equal where the subject is not exist between the X-ray tube and the X-ray detector.
Sliced units of a subject body are called "slices," and the thickness of each slice at the time of obtaining a single reconstruction image is called "slice thickness." If the detection elements constituting a detector are aligned in a line, the tomographic image of the subject body detectable at a time is a single slice, and such a detector is called a single-slice type detector. Recently, the use of a multi-slice type detector which comprises multiple lines of detection elements and can acquire multiple slices of projection data at a time has begun in order to shorten the scanning time. In the multi-slice type detector, however, it is not sufficient to correct the outputs of the main detectors in accordance with the output of the sub detector to obtain a reconstruction image with high precision.
In general, when the electron beam emitted from the cathode of the X-ray tube is incident on the side surface (inclined surface) of the rotary anode having a conical shape, X-ray beam is generated in the direction that intersects the irradiation direction of the electron beam at 90 degrees. The point of collision of the electron beam on the rotary anode is the focal point of the X-ray tube. When this point of collision of the electron beam onto the inclined surface of the rotary anode is shifted, the position of generation of the X-ray beam or the focal point is shifted in the axial direction of a subject body (hereinafter called "body-axis direction"). The shifts of the rotary anode, the cathode and the flying direction of the electron beam result in the shifting of the focal point. Since the position shifting occurs due to a temperature change, a chronological change, the wearout of the electrodes and the like, it is inevitable that the focal position varies finely during the acquisition of projection data, changing the intensity profile of the X-ray beam in the slice direction.
If the intensity profile of the X-ray beam in the slice direction varies, the amount of X-ray beam incident to a specific line of multi-slice type detection elements may increases or the amount of X-ray beam may decrease for another line of detection elements. Even if the variation in the line of detection elements is corrected by using the sub detector, the variation in the lines is present. In the CT apparatus using a multi-slice type detector, to reduce the effective slice thickness of a reconstruction image, not only projection data from the detection elements associated with the corresponding slice but also projection data from the detection elements associated with different slices may be used to reconstruct the single slice of interest. If the output of a specific line of the detection elements is too large or too small, even when the sensitivity profile of the detector in the direction of the body-axis direction (slice direction) does not vary, an artifact (false image) is produced in the reconstruction image. The artifact reduces the quality of the reconstruction image and presents a poor image. Therefore, there is a strong demand for some ways of overcoming the conventional shortcoming of CT apparatuses using a multi-slice type detector, which originates from the shifting of the focal point, and improving the quality of reconstruction images.